Preheat the grill to medium heat (about 350°F to 400°F).
In a large skillet, heat 1 tablespoon of olive oil over medium heat.
Add the chopped onion and minced garlic to the skillet and sauté for 3-4 minutes until softened.
Add the ground beef to the skillet and cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes.
Stir in the cooked rice, drained diced tomatoes, dried oregano, ground cumin, salt, and black pepper. Cook for another 2 minutes to combine flavors, then remove from heat.
Stir in half of the shredded cheddar cheese into the beef and rice mixture.
Brush the outside and inside of each bell pepper with the remaining 1 tablespoon of olive oil.
Stuff each bell pepper with the beef and rice mixture, pressing down gently to fill completely.
Place the stuffed peppers upright on the grill grates and cover the grill.
Grill for 20-25 minutes, turning occasionally, until the peppers are tender and slightly charred.
In the last 5 minutes of grilling, sprinkle the remaining shredded cheddar cheese on top of each stuffed pepper and close the grill lid to melt the cheese.
Remove the stuffed peppers from the grill and let them rest for 5 minutes before serving.